1. Field of the Invention
The invention relates to a method of optically scanning an information plane of a record carrier by means of a scanning beam which is focused to form a scanning spot on the information plane, the scanning spot and the record carrier being moved relative to each other so that it scans such plane. The invention also relates to a record carrier, to a record carrier master which is used in the manufacture of such as record carrier, and to an optical scanning apparatus, all of which are suitable for use of the method.
Such method can be used for examining objects by means of a scanning optical microscope, but it is particularly suitable for scanning an optical record carrier having recorded thereon an audio or video program or digital data.
The information plane of a record carrier may be a plane already recorded with optically readable information on a plane in which information is to be inscribed. In the former case the scanning beam is a read beam and in the latter case the scanning beam is a write beam modulated with the information to be inscribed. The record carrier may be a round disc-shaped record carrier or a tape-shaped record carrier.
2. DESCRIPTION OF THE RELATED ART
Nowadays such record carriers and the associated read apparatus are used in large numbers. In addition to the robustness of disc-shaped record carriers, the large storage capacity is also an important factor which has contributed to the success of the optical recording systems. The optical disc having a diameter of 12 cm, which is known under the name of "Compact Disc" or "CD", may comprise, for example a digitised music program of one hour, whilst the larger optical disc having a diameter of 30 cm, which is known under the name of "Laservision Disc" may comprise a video program of one hour.
However, there is an ever increasing need of recording still larger quantities of information on such record carriers. While maintaining the diameter of present-day optical discs, which are easy to handle, this means that the information areas such as the known pits in the "CD" and "Laservision" discs and in the optical discs for digital data storage, or the magnetic domains in the magneto-optical discs which are inscribable and erasable must be smaller. The current techniques provide the possibility of inscribing such smaller information areas. However, reading these smaller areas is a problem.
In the present-day read apparatus a read objective lens system having a numerical aperture, NA, of the order of 0.45 is used. Semiconductor diode lasers such as an AlGaAs laser emitting radiation at a wavelength of the order of 800 nm are used as radiation sources. This radiation can be focused by the read objective to a diffraction-limited radiation spot whose half-value width (FWHM) is of the order of 1 micron. The half-value width is understood to mean the distance between the points in the radiation spot where the intensity is half the maximum intensity in the centre of the radiation spot. Information areas having a width of the order of 0.5 micron and an average length of the order of 1 micron can be well-separated read by means of this scanning spot.
In this reading operation use is made of diffraction of the read beam by the information areas. At the stated values of the wavelength, the numerical aperture and the size of the information areas, the information structure can be considered to be a two-dimensional diffraction grating which splits the incident radiation beam into a non-diffracted zero-order sub-beam, a plurality of diffracted first-order sub-beams and a plurality of sub-beams diffracted in higher orders. The zero-order sub-beam has a constant phase and amplitude and is not influenced by a movement of the information structure with respect to the scanning spot. However, the phase or amplitude of the first-order sub-beams is dependent on the movement of the information structure.
An objective concentrating the zero-order sub-beam and a part of the first-order sub-beams from the record carrier on a detector is arranged between the record carrier and a radiation-sensitive detector. When moving the information structure and the scanning spot with respect to each other, the phase of a first-order sub-beam varies with respect to that of the zero-order sub-beam. By interference of a portion of a first-order sub-beam with the zero-order sub-beam, the intensity of the radiation received by the detector varies. This intensity variation represents the information which has been read, i.e. the succession of information areas in the scanning direction.
The angles at which the first-order sub-beams and the higher-order sub-beams are diffracted depend on the local period of the information structure, i.e., the distance between the beginning of a first information area and that of a subsequent information area. In the case of a decreasing period, or an increasing spatial frequency, the diffraction angles become larger until (,at a given spatial frequency referred to as the "conventional" cut-off frequency,) the first-order sub-beams entirely fall outside the pupil of the objective. Information areas whose spatial frequency lies above this cut-off frequency can therefore no longer be detected.
To be able to read an information structure at a spatial frequency above the said conventional cut-off frequency, U.S. Pat. No. 4,242,579 issued Dec. 30, 1980, assigned to the present assignee, proposes to arrange the said objective, referred to as the observation objective in such Patent, asymmetrically in the zero-order sub-beam coming from the record carrier. Since the observation objective is displaced in the direction in which one of the first-order sub-beams is diffracted, this sub-beam is captured also at higher spatial frequencies. This beam interferes in the detector plane with a part of the zero-order sub-beam, resulting in an interference pattern. The variation in this pattern, representing the information which has been read, is detected by means of a detector whose width in the scanning direction is smaller than the period of the interference pattern. In the apparatus according to U.S. Pat. No. 4,242,579 stringent requirements are imposed on the alignment of the optical elements, notably if a reflecting information structure is read and a transparent protective coating is present on this structure. In practice the cut-off frequency can be increased to approximately 1.5 times the conventional cut-off frequency with this apparatus.